1. Field of the Invention
The present invention relates to an optical transmission system that transmits an OTDM signal (optical time division multiplexed signal) via an optical transmission fiber and one or both of an optical linear repeater and an optical regenerator repeater, and wherein a control signal carrying transmission quality monitoring information, frame information, multiplexed signal channel information, etc., is transmitted by being multiplexed with an OTDM signal.
In addition, the present invention relates to an optical transmission system that transmits an optical signal via an optical transmission fiber and optical linear repeaters, and wherein monitor light used in wavelength dispersion compensation of the transmission path is transmitted by being multiplexed with signal light.
2. Background Art
In an optical transmission system, transmission quality monitoring, frame synchronization, and extracting multiplex signal channels are very important. In conventional electrical time division multiplexing (ETDM) that multiplexes channels with a plurality of lines at the electrical stage, the transmission quality monitoring information, frame information, and multiplexed signal channel information corresponding to each of these functions are accommodated in the overhead of the SDH frame, and by electrical signal processing after conversion of the signal light into an electrical signal by a light receiver, transmission quality monitoring, frame synchronization, and channel extraction are carried out. In addition, a method of improving transmission characteristics consists in lowering the error rate by adding a forward error correction code (see M. Tomizawa et al., xe2x80x9cSTM-64 linearly repeating optical transmission experiment using forward error correcting codesxe2x80x9d, in Electron. Lett., Vol. 31, No. 12, pp. 1001-1003, 1996.)
In contrast, in optical transmission systems, one method of improving the transmission speed is optical time division multiplexing (OTDM), which multiplexes a plurality of an optical short pulses while offsetting the timing along the time axis. Furthermore, for optical time division multiplexing, there is the parallel form shown in FIG. 18 (see Japanese Unexamined Patent Application, First Publication, No. Hei 10-229364, xe2x80x9cOptical Pulse Multiplexing Apparatusxe2x80x9d) and the serial form shown in FIG. 19 (see S. Kawanishi et al., xe2x80x9cAll-optical time-division-multiplexing of 100 Gbit/s signal based on four-wave mixing in a travelling-wave semiconductor laser amplifierxe2x80x9d, Electron. Lett., Vol. 33, No. 11, pp. 976-977, 1997).
In FIG. 18, the optical pulse train of the repetition rate f0 is split into N parts by an optical splitter 61, and input into respective optical modulators 62-1xcx9c62-N. The optical signals modulated by each optical modulator are respectively amplified by optical amplifiers 63-1xcx9c63-N, have a different delay imparted by optical delay devices 64-1xcx9c64-N, and are coupled by an optical coupler 65. Thereby, an OTDM signal having a bit rate of Nf0 is generated. When the bit rates of all the lines are equal, this structure can generate an OTDM signal that time division multiplexes N lines of optical signals having arbitrary bit rates by respectively multiplying the optical pulse train having a fundamental frequency f0 split into N parts, in the case that the N lines of a modulated signal having has a bit rate mif0 (i=1, 2, . . . , N, and mi is an integer equal to or greater than 1).
In FIG. 19, an optical pulse train having a repetition rate xcexa3mif0 is generated by a high speed optical pulse train generating means 66, and modulated by an optical modulator 67-1 using a modulation signal having a bit rate of m1f0, and amplified by optical amplifier 68-1. In the following manner, it is possible to generate an OTDM signal that has a time division multiplexed signal light of N lines by sequential modulation using modulation signals respectively having bit rates mif0 with each modulator.
In addition, in optical transmission systems, one of the main factors causing deterioration of transmission characteristics is the wavelength dispersion of the optical transmission path. When this wavelength dispersion is large, because the waveform of the signal light is distorted, inter-symbol interference causes bit errors. The influence of this increases as the transmission speed increases. Therefore, when constructing an optical transmission system, it is necessary to understand the wavelength dispersion characteristics of the optical transmission path and carry out dispersion compensation.
A conventional means for measuring wavelength dispersion when implementing a system, as shown in FIG. 20, is measuring the zero-dispersion wavelength of the optical transmission path using a PM-AM converter, and using this to determine the amount of wavelength dispersion (see M. Tomizawa, et al., xe2x80x9cNonlinear influence on PM-AM conversion measurement of group velocity dispersion in optical fibersxe2x80x9d, Electron. Lett, Vol.30, No.17, pp. 1434-1435,1994).
In FIG. 20, CW light having a wavelength xcex1 output from the monitor light generation means 71 of the optical transmitter is input into an optical phase modulation means 72, and monitor light having applied a phase modulation of frequency xcfx891 is sent to the optical transmission fiber 73. The monitor light is sent via an optical transmission fiber 73 and an optical linear repeater 74, amplified by the optical amplifier 75 of the optical receiver, and received by an optoelectric conversion means 76. At this time, due to the wavelength dispersion that the monitor light undergoes in the optical transmission fiber 73, an intensity modulation component of frequency xcfx891 depending on phase modulation appears.
Because this intensity amplitude depends on the amount of wavelength dispersion the light of frequency xcex1 receives over the entire optical transmission fiber, if intensity amplitude information from an electric signal output from the optoelectric conversion means 76 of the monitor clock detection means 77 is extracted, the average amount of the wavelength dispersion over the entire optical transmission fiber can be known. This information is fed back to the monitor light generation means 71 using the wavelength dispersion compensation amount control means 78, and by carrying out the same measurement a number or times by changing the wavelength of input light, it is possible to set an arbitrary wavelength that is suited for the optical transmission fiber that has been introduced into the system. Normally, by matching the average zero dispersion wavelength of the optical transmission fiber as a whole, it is possible to minimize the amount of dispersion of the optical transmission fiber.
However, in high speed transmission systems of 40 Gbit/s or greater, adjusting dispersion equalization that optimally compensates the wavelength dispersion in real time is necessary as a measure against time dependent fluctuation of the wavelength dispersion due to temperature fluctuation. As a conventional applied dispersion equalization method used while a system is in operation, a method, as shown in FIG. 21, has been proposed in which monitor light having a wavelength differing from that of the signal light is wavelength multiplexed with the signal sight and transmitted, the monitor light only is separated by a wavelength filter from the wavelength multiplexed light optically split at the receiver, and the amount of wavelength dispersion is measured (see Kuwahara et al., xe2x80x9cStudy of adjusting dispersion equalization by dispersion fluctuation detection using the PM-AM conversion effectxe2x80x9d, Electronic Information Communication Association Technical Research Report OCS 98-5 [in Japanese]).
In FIG. 21, the signal light generation means 81 outputs a signal light having a wavelength xcex0. The monitor light generation means 82 outputs a CW light having a wavelength of xcex1 (xe2x89xa0xcex0), and the optical phase modulation means 83 generates monitor light by applying a phase modulation of frequency xcfx891 to the CW light. The signal light and the monitor light are multiplexed by the optical coupling means 84, and transmitted via an optical transmission fiber 85 and an optical linear repeater 86. At this time, the pulse width of the signal light broadens due to the wavelength dispersion received from the entire optical transmission fiber, and an intensity modulation component having frequency xcfx891, which depends on the phase modulation, appears in the monitor light.
The transmitted signal light and monitor light are amplified by the optical amplifier 87 of the optical receiver, and split into two parts by the optical splitting means 88. One of the parts is input into the optical band pass filter 89 that passes a wavelength xcex1 and thereby only the monitor light is extracted, while the signal light is transmitted. This monitor light is converted into an electrical signal by the optoelectric conversation means 90. Because the intensity amplitude of the monitor light is dependent on the amount of wavelength dispersion the light of wavelength xcex1 receives over the entire optical transmission fiber, if intensity amplitude information is extracted from the electric signal output from the optoelectric conversion means 90 at the monitor clock detection means 91, the amount of the average wavelength dispersion of the entire optical transmission fiber can be known. This information is fed back to the monitor light generation means 82 using the wavelength dispersion compensation amount control means 92, and by carrying out the same measurement a number or times by changing the wavelength of input light, it is possible to set an optimal transmission wavelength by tracking the zero dispersion wavelength fluctuation while the system is in operation.
In this connection, in the conventional OTDM format, an optical pulse signal is only multiplexed and dispersed on a time axis, and does not have the overhead that ETDM does. Therefore, there has been almost no study of monitoring transmission quality and frame synchronization, extraction of multiplexed channels, or the transmission of forward error correcting codes.
In contrast, a method of transmitting forward error correction codes without using overhead or redundant bit sequences has been proposed. This method restricts the bit rate increase due to the foreword correction bit sequence of the main signal by wavelength multiplex transmission of the forward error correction code by a control signal having a wavelength that differs from the main signal (see Japanese Unexamined Patent Application, First Publication, No. Hei 11-32008, xe2x80x9cOptical Transmission Apparatusxe2x80x9d). For example, control information is carried and transmitted on the monitor light shown in FIG. 21, and when this method is used, forward error correction without overhead is possible for the OTDM format as well.
However, in the wavelength multiplex transmission of the main signal and the control signal, because two waves must transit the optical band pass filter provided in the optical linear repeater, compared to the bit rate of the main signal, a broad bandwidth is necessary. Due to this, there is the problem that the exclusion effect of the ASE (amplified spontaneous emission) the optical amplifier generates is reduced, and the transmission characteristics deteriorate. Furthermore, the faster the bit rate becomes, the larger the influence of the wavelength dispersion of the optical transmission fiber becomes, and the phase difference between the two waves of the main signal and the control signal exceeds one bit. Thus there is the problem that correct forward error correction cannot be carried out.
It is an object of the present invention to provide an optical transmission system that can transmit control signal light corresponding to the overhead accommodating the transmission quality monitoring information, frame information, multiplexed signal channel information, etc., at the same (or nearly the same) wavelength as the OTDM signal.
In addition, as shown in FIG. 21, in an optical transmission system that transmits by wavelength multiplexing a signal light having a wavelength of xcex0 and monitor light having a wavelength of xcex1 and carries out wavelength dispersion compensation using the monitor light, the wavelength dispersion compensation is always to the monitor light wavelength, the offset of the amount of wavelength dispersion of the monitor light wavelength and the signal light wavelength is known in advance, and based on this, control of the signal light wavelength of the signal light generation means 81 was necessary.
In addition, in order to wavelength multiplex and transmit the signal light wavelength and the monitor light having a difference wavelength, as described above, wide band characteristics were required of the optical linear repeater 86. Due to this, the influence of ASE generated by the optical amplifier becomes large, and there is the concern of the deterioration of the signal characteristic.
It is an object of the present invention to provide an optical transmission system that monitors the amount of wavelength dispersion using monitor light having the same (or nearly the same) wavelength as the signal light, and always carries out optical wavelength dispersion compensation.
In order to solve the above-described problems, the first aspect of the invention is characterized in an optical transmitter providing: an optical time division multiplexed signal generating means that outputs an OTDM signal having a wavelength xcex0 that has a time division multiplexed signal light having N lines and having a bit rate mif0 that is an integral multiple of the fundamental frequency f0, a control light generation means that generates control signal light wherein an optical pulse train, having a wavelength xcex1 equal or almost equal to the wavelength of the OTDM signal and having a bit rate kf0 that is an integral multiple of the fundamental frequency f0 or a bit rate f0/k that is a unit fraction of an integral number of the fundamental frequency f0 that are synchronized with the OTDM signal, is modulated by control information of said OTSM signal, an optical pulse broadening means that broadens the optical pulse waveform of the control signal in a time range, and outputs a control signal having an optical peak intensity set sufficiently low in comparison to the OTDM signal peak intensity, and an optical coupling means that multiplexes the OTDM signal and the control light and delivers them to the optical transmission fiber. Furthermore, the first invention is characterized in an optical receiver providing: a light splitting means that splits into two parts the transmitted light, an optical pulse narrowing means that restores the control light induced in a part of the light split by the optical splitting means to the original optical pulse train, an optoelectric conversion means that converts the output light of the optical pulse narrowing means to an electric signal, a control signal processing means that extracts from the electrical signal information relating to the optical time division multiplexing of the OTDM signal and control information such as the timing clock, etc., and an optical time division multiplexing means that, depending on the control information, carries out optical time division demultiplexing of the OTDM signal included in other portion of light split into an N line signal light by the optical splitting means.
The optical transmission system of the first aspect of the invention can transmit control signals corresponding to the overhead accommodating transmission quality monitoring information, frame information, multiplexed signal channel information, etc, at the same or nearly the same wavelength as the OTDM signal. In addition, using an optical linear repeater or an optical regenerator repeater, it is possible to use an optical band pass filter having the minimal necessary bandwidth through which the OTDM signal and the control light can pass, and it is possible to realize transmission characteristics that are little influenced by the ASE generated by optical amplifiers.
Furthermore, for each optical linear repeater, by improving the S/N ratio of the OTDM signal and the control signal, it is possible to reduce the influence of non-linear optical effects such as self-phase modulation and inter-bit four wave mixing light, and it is possible to implement long distance transmission.
In addition, the first aspect of the invention is characterized in providing in a part or all of a plurality of optical linear repeaters: an optical splitting means that splits transmitted light into two parts, a first optical signal regeneration means that regenerates the OTDM signal included in one part of the light split by the optical splitting means, an optical pulse narrowing means that restores the control signal included in the other part of the light split by the optical splitting means to the original optical pulse train, a second optical pulse regenerating means that regenerates the optical pulse train of the control light output from the optical pulse narrowing means, an optical pulse broadening means that broadens the optical pulse waveform of the regenerated control light in a time range, and outputs the control light having an optical peak intensity set sufficiently low compared to the OTDM signal peak intensity, and an optical coupling means that multiplexes the regenerated OTDM signal and the control light and delivers them to the optical transmission fiber.
The optical transmission system of the second aspect of the invention is characterized in an optical transmitter providing: an signal light generation means that generates signal light having a wavelength xcex0 and a bit rate f0, a monitor light generating means that generates monitor light of an optical pulse train having a wavelength xcex1 equal to or almost equal to the wavelength of the signal light and having a repetition rate f1, an optical pulse broadening means that outputs monitor light having an optical peak intensity set sufficiently low compared to the signal light peak intensity, and an optical coupling means that multiplexes signal light and monitor light output from the optical pulse broadening means and delivers the monitor light to the optical transmission fiber. Furthermore, the optical transmission system of the second invention is characterized in an optical receiver providing: a wavelength dispersion adjustment means that provides wavelength dispersion to the delivered light, an optical splitting means that splits into two parts the output light of the wavelength dispersion adjustment means and outputs one part to a signal light processing means that carries out reception processing of the signal light, an optical pulse narrowing means that restores the monitor light included in the other part of the light split by the optical splitting means to the original optical pulse train, an optoelectric conversion means that converts the output light of the optical pulse narrowing means into an electric signal, a monitor clock detection means that detects the clock component of the monitor light from the electric signal, and a wavelength dispersion compensation amount control means that controls the wavelength dispersion compensation amount of the wavelength dispersion adjustment means so that the clock component is maximized.
According to the second aspect of the invention, it is possible to monitor the wavelength dispersion compensation amount using monitor light having the same or nearly the same wavelength as the signal light, and always maintain it at the optimal value. Furthermore, with an optical linear regenerator it is possible to use an optical band pass filter having the lowest necessary bandwidth to transmit the signal light and the monitor light, and it is possible to realize transmission characteristics that are little influenced by the ASE generated by optical amplifiers.
In addition, the second aspect of the invention is characterized in a structure in which a wavelength dispersion compensation amount control means controls the wavelength of the signal light generating means and the monitor light generating means of the light transmitter so that the clock component detected by the monitor detection means is largest, instead of providing a wavelength dispersion adjustment means in the optical receiver.
Furthermore, the second aspect of the invention is characterized providing in a part or all of a plurality of optical linear repeaters: a wavelength dispersion adjustment means that provides wavelength dispersion to the delivered light, an optical splitting means that splits into two parts the output light of the wavelength dispersion adjustment means and outputs this part to the optical transmission fiber, an optical pulse narrowing means that restores the monitor light included in the other part of the light split by the optical splitting means to the original optical pulse train, an optoelectric conversion means that converts the output light of the optical pulse narrowing means to an electric signal, a monitor clock detection means that detects the clock component of the monitor light from the electric signal, and a wavelength dispersion compensation amount control means that controls the wavelength dispersion compensation amount of the wavelength dispersion adjustment means so that the clock component is maximal.